1. Field of the Invention
The present invention deals with NO.sub.x, SO.sub.x and particulate removal systems for fossil fuel burning boilers in general and more particularly to such systems utilizing ammonia reagents therein.
2. Description of the Related Art
The by-products of combustion are stack gases, which are present with all fossil fuels, and ash, which is present in substantial quantity with coal and in lesser quantity with oil combustion.
Stack gases contain particulate matter as well as certain gaseous products of combustion which produce air pollution if discharged in sufficient quantity. Stack gases contain carbon monoxide and carbon particles. The problems with stack gases arise principally from fly ash and oxides of sulfur and nitrogen.
Stacks are used successfully for dispersing gases and suspended particulate matter over a large area. Their heights have increased as unit sizes have increased. Stacks as high as 1200 ft are used, particularly in narrow valleys where it is desired to disperse gases beyond surrounding hills.
While stacks are effective in dispersing gases, in locations where there is a concentration of industry the atmosphere can become overloaded with the discharge from many stacks during periods of air stagnation. The effect is particularly objectionable on damp, foggy days when a combination of smoke and fog-blankets the area. Thus, when the total discharge of pollutants reaches a certain amount, the stack alone may not constitute an adequate provision for the health and comfort of the community and hence particular equipment for removing particulates, nitrogen oxides (NO.sub.x), and sulphur oxides (SO.sub.x) are required.
The problems of particulate or ash removal and disposal are significant principally in the case of fuels such as coal. Electrostatic precipitators and other particulate collection devices have been used to remove such particulates from the stack.
Particulate removal is especially needed with pulverized-coal firing boilers since all the burning is accomplished in suspension with the result that about 80 to 90% of the ash remains in the flue gases.
To meet the objective of a clear stack, high efficiency particulate collection devices are now generally required to remove the fly ash from flue gases from units where fuels are burned in suspension. Electrostatic precipitators are the most widely used and preferred particulate collector.
Electrostatic precipitators produce an electric charge on the particles to be collected and then propel the charged particles by electrostatic forces to the collecting curtains. The precipitator operation involves four basic steps:
1. An intense, high voltage electrical field is maintained between the discharge electrode and the collecting curtains. PA1 2. The carrier gases are ionized by the intense, electrical field. These gas ions, in turn, charge the entrained particles. PA1 3. The negatively charged particles, still in the presence of an electrostatic field, are attracted to the positively (grounded) charged collecting curtains. PA1 4. The collected dust is discharged by rapping into storage hoppers.
The collection efficiency of the electrostatic precipitator is related to the time of particle exposure to the electrostatic field, the strength of the field, and the resistivity of the dust particle. An efficiency in excess of 99% is obtained at a cost generally favorable in comparison with other types of equipment.
This technology has been advanced through flue gas conditioning for the marginally designed precipitators to meet the current low emission standards (0.03-0.1 lb/10.sup.6 Btu) and to enhance particulate collection. Typically, sulfur trioxide (SO.sub.3) and/or ammonia (NH.sub.3) can be injected into the precipitator without extensive modification. Uniform NH.sub.3 injection control and flue gas flow distribution across the precipitator are important to reduce particulate emissions. For coal fired units, 2-10 ppm NH.sub.3 level is normally used in flue gas to condition the flue gas and particulate while avoiding excessive NH.sub.3 build up in the collected fly ash for disposal. This ammonia injection results in greater particulate removal as well as some SO.sub.x desulfurization due to reaction with SO.sub.3. For oil fired units, NH.sub.3 reacts with SO.sub.3 to form particulates and prevent acid mist at stack exit.
Electrostatic precipitators are normally located downstream of the air heater and the SCR (selective catalyst reduction) unit. This unit is used to remove NOx and uses ammonia in conjunction with a catalyst bed to effect the NO.sub.x reduction.
The SCR catalyst is typically plate type or honeycomb type developed and marketed commercially. The main reaction in the DeNO.sub.x process is: EQU NO+1/4O.sub.2 +NH.sub.3 .fwdarw.N.sub.2 +3/2H.sub.2 O.
To remove SO.sub.2 generated in the combustion process, the flue gas is treated in plate/spray type absorbers known as Flue Gas Desulfurization (FGD) units to reduce SO.sub.2 to the required level (80-95% reduction). Such units use lime and sometimes ammonia spray solutions to affect the desulfurization.
Currently, the Walther Process employs ammonia FGD followed by SCR DeNO.sub.x removal. In the prior art Walther process the SCR is located downstream from the FGD scrubber.
The sequence of NO.sub.x and SO.sub.x removal and the use of electrostatic precipitation varies in known processes. Thus it will be seen that although it is known in the prior art to use independently ammonia injection in SCR, electrostatic precipitation and FGD technology, to date there has been no systems that utilize all three systems sequentially integrated together in a particular order and arrangement with ammonia supplied to all three systems by a single controlled ammonia source.